The present invention relates generally to controlling burner fan control for a combination boiler. More particularly, the present invention relates to suitably initializing, modifying, or controlling the firing rate of an input fan of a combination boiler for a Domestic Hot Water (DHW) demand based on an estimated DHW flow rate, a DHW set point, and an error in a DHW output temperature.
Current combination boiler implementations suffer drawbacks associated with initially and continuously undershooting and overshooting heated water temperatures when attempting to provide DHW at a desired set point temperature. One attempted solution is to provide a DHW output flow sensor within a combination boiler to determine a DHW output flow rate and to use the directly measured DHW output flow rate to adjust a boiler loop temperature to compensate for the DHW output flow rate. However, providing a DHW flow sensor adds both cost and complexity to a combination boiler. Furthermore, flow sensors typically have a minimum flow rate detection threshold, below which the flow sensor does not detect a current flow rate. Thus, low DHW output flow rates are not detected and heated DHW output may be significantly delayed or DHW output may be concluded before heated water is provided.
Problems also arise with combination boilers that initialize a burner input rate (e.g., fan speed) only on a proportional term. For example, if a DHW output temperature is close to a set point temperature when the burner fires, an input fan of the burner may initialize at a low input rate, causing a significant DHW output temperature undershoot. The combination burner may significantly overshoot the DHW output temperature when there is a low DHW output flow rate or when the initial DHW output temperature is significantly lower than the set point temperature.
It would therefore be desirable for a combination boiler to provide heated water as quickly as possible with minimal overshoot or undershoot of a DHW output set point temperature.